mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Lateral Gene Transfer between Archaea and Bacteria
Thermatoga maritima ''belongs to the Thermatogales and is a rod shaped bacterium with an optimal growing temperature of 80°C. This is not surprising given that it was first isolated from geothermal marine sediments in Vulcano, Italy (1). Study of the small subunit ribosomal rRNA phylogeny reveals that ''T. maritima is one of the slowest evolving Eubacteria (1). T. maritima's genome is a single chromosome with ~1.8 Mbp and an average GC content of 46% (1). Lateral Gene Transfer between Archaea and T. maritima Lateral gene transfer is the movement of genetic material between species. It is a major mechanism for the evolution of the prokaryotic genome. Gene transfers often facilitate the acquisition of functions that allow otherwise incapable organisms to survive in new environments. In the case of T. maritima, which has 1,877 predicted coding sequences, 52% are similar to eubacteria, 21% are shared by Bacillus subtilis, 15% by Aquifex aeolicus and 21% by archaeal species (1). To highlight the true magnitude of the genes shared between Archaea and T. maritima, A. aeolicus shares 16% with archaeal species while B. subtilis ''shares only 7% (1). Though there are many explanations for the increased presence of archaea-like genes in the ''T. maritima genome, including the sharing of a common ancestor, Nelson et al. ''(1999) presented another hypothesis. They put forward the idea that this was due to lateral gene transfer between archaeal species and ''T. maritma. Of the 24% of T. maritima genes that are of archeal origins, about half of them can be traced to Pyrococcus horikoshii. P. horikoshii is a hyperthermophylic extremophile that lives at depths of 1395 metres with an optimal temperature of 98°C (2). It is argued however that the exceptionally large overlap between the T. maritima and archaeal genomes, is the result of shared ancestry of portions of the genome as a result of lateral gene transfer (1 ). Evidence for this can be seen in the fact that the archaea-like genes T. maritima has are not distributed evenly between the different biological role categories, but are clustered as you would expect from a lateral transfer (1 ). For example, 60% of the electron transport proteins and 49% of the transport proteins are shared with archaea while the majority of the housekeeping genes are common to eubacteria (1 ). The T. maritima ''genome is clustered in 15 regions of the chromosome, ranging in size from 4-20 kb. The genes, and conservation of gene order in 7 of these regions, have only been described in the genome sequences of thermophilic archaea. Along with those 7 regions, two of the clustered regions are associated with 30 bp repeat elements found only in archaea and ''T. maritima. ''It has been hypothesized that these 30 bp repeats are important for lateral gene transfer. Thermatoga-genome.jpg|Map of the ''T. maritima genome. Red donut denotes Archaea-like islands within the genome (1). top4t.maritima,archaeagenes.gif|Table showing the top four matches for T. maritima genes by role (1). Thermatoga archaea genes.gif|Table showing genes that T. maritima shares with archaea and hyperthermophiles (1). Nelson et al. (1999), were able to present evidence for lateral gene transfer between archaea and T. maritima, because they fully sequenced the T. maritima genome. This afforded a view of the genome as a whole that allowed them to compare it to previously sequenced archaeal genomes (1). Chi squared analysis of the T. maritima genome, also supported the idea of lateral gene transfer, revealing that there were 51 regions within the T. maritima genome that are not uniform with the other regions (1). Conclusions This study, one of the first for T. maritima, highlights the fact that lateral gene transfer occurs more frequently than previously thought. The transfer of genetic material between species may be 'easiest' in bacteria and archaea, but it is definitely not limited to them. Gene transfer also occurs in plants and in humans. Other studies of T. maritima followed that focused on individual pathways and the function of the archaeal derived proteins in those pathways. References 1. Nelson KE'', et al.'' (1999) Evidence for lateral gene transfer between Archaea and bacteria from genome sequence of Thermotoga maritima. Nature 399(6734):323-329. 2. MicrobeWiki, (2010) Pyrococcus horikoshii , ''Microbewiki, ''http://microbewiki.kenyon.edu/index.php/Pyrococcus_horikoshii